Because the Long-Term Evolution (LTE) Standard Release 8 (hereinafter “Rel-8”) frame structure 2 (time-division duplex [TDD]) may have many more downlink subframes than uplink subframes and because each of the downlink subframes carries up to two transport blocks, Rel-8 TDD supports transmission of up to 4 Ack/Nack (A/N) bits in a subframe. If more than 4 Å/N bits are required, the spatial bundling in which two Ack/Nack bits of the same downlink subframe are bundled is supported. These 4 Ack/Nack bits can be transmitted using channel selection. More recently, LTE Release 10 (hereinafter “Rel-10”) uses channel selection for up to 4 Ack/Nack bits to support carrier aggregation for both frame structures, i.e., frequency division duplex (FDD) and TDD. Therefore, the use of channel selection for Ack/Nack feedback is of growing interest.
Ack/Nack bits are carried in LTE, using physical uplink control channel (PUCCH) format “1a” and “1b” on PUCCH resources, as described below. Because no more than 2 bits can be carried in these PUCCH formats, 2 extra information bits are needed for carrying 4 Ack/Nack bits. These extra two bits can be conveyed through channel selection.
A user equipment (UE), sometimes hereinafter referred to as a “client node,” encodes information using channel selection by selecting a PUCCH resource to transmit on. Channel selection uses 4 PUCCH resources to convey these two bits. This can be described using the data in Table 1 below:
TABLE 1PUCCH format 1b channel selectionCodewords 0 to 15RResDRes 0000000100100011010001010110011110001001101010111100110111101111001j−j−10000000000001100001j−j−10000000022000000001j−j−10000330000000000001j−j−1
Each column of the table indicates a combination of Ack/Nack bits (or a “codeword”) to be transmitted. Each row of the table represents a PUCCH resource. Each cell contains a QPSK symbol transmitted on the PUCCH resource to indicate the codeword. The “DRes” column indicates which PUCCH resource carries the QPSK symbol, and the “RRes” column indicates the PUCCH resource used to carry the reference symbol. It is noted that the data and reference symbol resources are the same for Rel-8 channel selection. Note that each column of the table contains only one non-zero entry, since channel selection requires that only one resource is transmitted upon at a time on one transmission path. Transmitting on one transmission path maintains the good peak to average power characteristics of the signals carried on the PUCCH. The term “transmission path” refers to an RF chain that contains at least one power amplifier and is connected to one antenna.
For example, when Ack/Nack bits ‘0110’ are to be transmitted, the UE can transmit the QPSK data symbol ‘−j’ using PUCCH resource ‘1.’ The reference signal transmission can also be on PUCCH resource ‘1’.
LTE carries Ack/Nack signaling on format 1a and 1b of the physical uplink control channel (PUCCH), as specified in Rel 10. An example of the subframe structure of PUCCH formats 1a and 1b with normal cyclic prefix is shown in FIG. 1. Each format 1a/1b PUCCH can be in a subframe made up of two slots. The same modulation symbol “d” can be used in both slots. Without channel selection, formats 1a and 1b set carries one and two Ack/Nack bits, respectively. These bits are encoded into the modulation symbol “d,” using BPSK or QPSK modulation, depending on whether one or two Ack/Nack bits are used.
Each data modulation symbol, d, is spread with a sequence, ru,vα(n) such that it is by a 12 samples long, which is the number of subcarriers in an LTE resource block in most cases. (For example, those of skill in the art will understand that a Multimedia Broadcast multicast service Single Frequency Network (MBSFN) transmission can use 24 subcarriers in a resource block when the subcarriers are spaced 7.5 kHz apart.). Next, the spread samples are mapped to the 12 subcarriers the PUCCH is to occupy and then converted to the time domain with an IDFT. Since the PUCCH is rarely transmitted simultaneously with other physical channels in LTE, the subcarriers that do not correspond to PUCCH are set to zero. Four replicas of the spread signal are then each multiplied with one element of an orthogonal cover sequence wp(m), where mε{0, 1, 2, 3} corresponds to each one of 4 data bearing OFDM symbols in the slot. There are 3 reference symbols (R1, R2, and R3) in each slot that allow channel estimation for coherent demodulation of formats 1a/1b.
There can be 12 orthogonal spreading sequences (corresponding to ru,vα(i) with aε{0, 1, . . . , 11} indicating the cyclic shift) and one of them is used to spread each data symbol. Furthermore, in Rel-8, there are 3 orthogonal cover sequences wp(m) with pε{0,1,2} and mε{0, 1, 2, 3}. Each spreading sequence is used with one of the orthogonal cover sequences to form an orthogonal resource. Therefore, up to 12*3=36 orthogonal resources are available per each resource block of the PUCCH. The total amount of resources that can carry Ack/Nack is then 36 times the number of resource blocks (RBs) allocated for format 1/1a/1b.
Each orthogonal resource can carry one Ack/Nack modulation symbol “d,” and, therefore, up to 36 UEs may transmit an Ack/Nack symbol on the same OFDM resource elements without mutually interfering. Similarly, when distinct orthogonal resources are transmitted from multiple antennas by a UE, they will tend to not interfere with each other, or with different orthogonal resources transmitted from other UEs. When there is no channel selection, the orthogonal resource used by the UE is known by the eNB. As discussed below, in case of channel selection, a predetermined set of the information bits determines the orthogonal resource to be utilized. The eNB detects that set of the information bits by recognizing what orthogonal resource is carrying other information bits.
Orthogonal resources used for reference symbols are generated in a similar manner as data symbols. They are also generated using a cyclic shift and an orthogonal cover sequence applied to multiple reference signal uplink modulation symbols. Because there are a different number of reference and data modulation symbols in a slot, the orthogonal cover sequences are different length for data and for reference signals. Nevertheless, there are an equal number of orthogonal resources available for data and for reference signals. Therefore, a single index can be used to refer to the two orthogonal resources used by a UE for both the data and reference signals, and this has been used since Rel-8. This index is signaled in Rel-8 as a PUCCH resource index, and is indicated in the LTE specifications as the variable nPUCCH(1). The aforementioned LTE specifications include: (1) 3GPP TS 36.213 V10.1.0, “3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Evolved Universal Terrestrial Radio Access (E-UTRA); Physical Layer Procedures (Release 10)”, March, 2011; (hereinafter “Reference ‘1’) and (2) 3GPP TS 36.211 V10.1.0, “3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Evolved Universal Terrestrial Radio Access (E-UTRA); Physical Channels and Modulation (Release 10)”, March, 2011. (hereinafter “Reference ‘2’). This index indicates both the RB and the orthogonal resource used to carry data and reference signals, and the indexed resource is therefore referred to as a ‘PUCCH resource’ in 3GPP parlance.
One cyclic shift may be used to transmit all symbols in a slot (including both data and reference symbols) associated with an antenna. In this case, the value of a is constant over the slot. However, LTE Rel-8 also supports cyclic shift hopping, where a varies over the slot. Cyclic shift hopping transmissions are synchronized within a cell such that UEs following the cell-specific hopping pattern do not mutually interfere. If neighbor cells also use cyclic shift hopping, then for each symbol in a slot, different UEs in the neighbor cells will tend to interfere with a UE in a serving cell. This provides an “interference averaging” behavior that can mitigate the case where one or a small number of neighbor cell UEs strongly interfere with a UE in the serving cell. Because the same number of non-mutually interfering PUCCH resources are available in a cell regardless of whether cyclic shift hopping is used, PUCCH resource can be treated equivalently for the hopping and non-hopping cases. Therefore, hereinafter when reference is made to a PUCCH resource, it may be either hopped or non-hopped.
The PUCCH format 1a/1b structure shown in FIG. 1 varies, depending on a few special cases. One variant of the structure that is important to some Tx diversity designs for format 1a/1b is that the last symbol of slot 1 may be dropped (not transmitted), in order to not interfere with SRS transmissions from other UEs.
In LTE Rel-10, carrier aggregation up to 4 Ack/Nack bits may be indicated using channel selection. The PUCCH resource that a UE is to use may be signaled using a combination of implicit and explicit signaling. In this case, one or more resources are signaled implicitly using the location of the scheduling grant for the UE on the PDCCH of its primary cell (PCell), and one or more resources may be indicated using the Ack/Nack resource indicator (ARI) bits contained in the grant for the UE on the PDCCH of one of the UE's secondary cells (SCells). This is shown in FIG. 2. While it is not shown in FIG. 2, those of skill in the art will understand that it is also possible for all PUCCH resources to be allocated with implicit signaling. This occurs when a PDCCH scheduling PDCCH on SCell is transmitted on PCell with cross carrier scheduling.
UEs may be scheduled on a set of control channel elements (CCEs) that are specific to that UE only. This is indicated in FIG. 2 as the UE Specific Search Space (UESS). The UE Specific Search Space is normally different in each subframe.
LTE PUCCH resources can be implicitly signaled by the index of the first CCE occupied by the grant transmitted to the UE on the PCell PDCCH (labeled nCCE,i=M in FIG. 2). Up to two PUCCH resources may be determined this way in Rel-10. When two resources are implicitly signaled, the second PUCCH resource index is calculated using the next CCE after the first CCE detected by the UE (i.e., nCCE,i=M+1, as shown in the figure). As discussed in section 10.1 of 3GPP TS 36.213 V10.1.0, “3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Evolved Universal Terrestrial Radio Access (E-UTRA); Physical Layer Procedures (Release 10)”, March, 2011, the first and second implicit PUCCH resource indices are mapped from the first CCE index using nPUCCH,i(1)=nCCE,i+NPUCCH(1) and nPUCCH,i+1(1)=nCCE,i+1+NPUCCH′(1) respectively, and so they are adjacent resources. Due to the way PUCCH resources are indexed in LTE, this means that they will typically share the same PUCCH physical resource block (PRB) unless one of the two resources is near the first or the last resource in a PRB.
Because the UE Specific Search Space varies subframe by subframe, the PUCCH resource mapped to by its CCEs also varies. Therefore, the implicit resource can be in multiple different RBs depending on the subframe.
In LTE Rel-10, two bits of the PDCCH on the SCell are used as ARI bits. Also, up to two PUCCH resources are indicated by PDCCH of the SCell. This means that 4 combinations of PUCCH resources are indicated by ARI, and each combination comprises one or two PUCCH resources.
In contrast to implicit signaling, explicit PUCCH resources (of which one is addressed by the ARI) are semi-statically allocated to each UE, and therefore do not move between PUCCH RBs unless the UE is reconfigured using higher layer signaling. Since implicitly signaled PUCCH resource occupies different RBs on a subframe-by-subframe basis, but explicitly signaled PUCCH resource occupies the same RB until the UE is reconfigured, the explicit and implicit PUCCH resources will commonly not be in the same PUCCH RB.
The pairs of explicit resources corresponding to each Ack/Nack Resource Indicator (ARI) state are independently signaled such that they can be positioned anywhere in the PUCCH resource. This can be implemented using the RRC signaling of PUCCH-Config information elements as disclosed in section 6.3.2 of 3GPP TS 36.331 V10.1.0, “3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Evolved Universal Terrestrial Radio Access (E-UTRA); Radio Resource Control (RRC); Protocol specification (Release 10),” March, 2011. This means that the PUCCH resources can be, but are not necessarily, configured to be in the same PRB.
Space Time Resource Selection Diversity (STRSD) codes can encode part of their information by selecting the PUCCH resource used for the reference signal. This means that the data bearing OFDM symbols and reference signal bearing OFDM symbols can be in different PUCCH resource blocks. (This is not possible in LTE Rel-8, since the reference signal and data are always on the same PUCCH resource.) If the reference signal resource is in a different RB than the data resource, it can travel through a channel with a different response than the channel the data travels through. In that case, the reference signal may not allow good channel estimation for the data, leading to much higher error rates and poorer performance.